1. Field of the Invention
The present invention relates to an water-base ink achieving high print quality on plain, recycled, and coated paper, resulting in good glossiness on glossy paper, and having well-balanced gas fastness, and relates to an water-base ink set which will exhibit low light source dependence with regard to the colors of resultant printed matter.
2. Description of Prior Art
An inkjet recording method is a method for discharging ink droplets from microscopic nozzles to record letters and graphics on a surface of a recording medium. Practical methods using such inkjet recording method include a method for converting electrical signals into mechanical signals using electrostrictive elements and intermittently discharging ink loaded in a nozzle head so that the letters and symbols are recorded on a surface of a recording medium, and a method for rapidly heating some portion of an ink closest to a discharging part of the nozzle head to generate foam and intermittently discharge the ink due to volumetric expansion so that letters and marks are recorded on a surface of a recording medium.
Inks to be used in these inkjet recording methods are required, when printing on a recording medium, that is paper, to resist smearing, have good drying characteristics and properties so that adjacent colors do not mix during multicolor printing such as color printing, as well as to have gas fastness and light fastness, and to exhibit low light source dependence with regard to the colors of the resultant printed matter.
Examples of means to satisfy such requirements are described in Japanese Patent Laid-Open Publication No. H10-025440, which discloses a method to improve light fastness by using pigment blue 15:3, and in Japanese Patent Laid-Open Publication No. H11-275376 which describes color-management methods.
For conventional color ink sets using pigments, a combination of pigment blue 15:3, pigment red 122, and pigment yellow 74 has been commonly used.
However, there is a problem with the combination of pigment blue 15:3, pigment red 122 and pigment yellow 74. It causes the phenomenon where the printed colors significantly differ according to the light source used when viewing the printed matter (light source dependence). “Light source dependence” refers to the phenomenon where the same printed colors looks different according to the light source used when viewing the printed matter. For example, a color looks different when viewed under the sun and when viewed under a fluorescent light. Typically, inks causing the phenomenon called metamerism tend to have light source dependence.
Neutral colors such as gray and khaki that are formed in the process called subtractive color mixture using cyan, magenta, and yellow, tend to have light source dependence. In the case of using inkjet inks, for example, when a gray color is made by combining pigment blue 15:3, pigment red 122, and pigment yellow 74, differences are made in the gray color according to the light sources used (light source dependence). The gray color looks yellowish-gray under the sun light and it looks bluish-gray under a fluorescent light. This phenomenon also occurs when combining pigment blue 15:4, pigment red 122 and pigment yellow 74.
With inkjet recording methods, there is another problem that it is impossible to achieve glossiness on glossy paper when the pigments have large particle diameters. Accordingly, it is preferable to use, in an inkjet recording method, pigments having small particle diameters so that glossiness can be achieved on glossy paper. However, when a copper phthalocyanine pigment having a small particle diameter is used as an ink for the inkjet recording, and when the printed matter is exposed to the atmosphere for a long period of time, only the cyan color composed by the copper phthalocyanine pigment deteriorates and discolors. This is because the copper phthalocyanine pigment has a low gas fastness and a poor balance with other colors (such as magenta, yellow, and black).
The present invention solves these problems. Therefore, it is an object of the present invention to provide an water-base ink achieving a high print quality on plain, recycled, and coated paper, achieving glossiness on glossy paper, and having a well-balanced gas fastness, and to provide an water-base ink set which will exhibit low light source dependence with regard to the color of the resultant printed matter.